1. Field of the Invention
The present invention relates to a comparator, and more especially, to a comparator with a self-biased reference voltage for an oscillator.
2. Background of the Related Art
Recently, due to the rapid progress of mobile communication apparatuses such as the cellular phones, apparatuses require many additional functions including temperature compensation, size reduction, and frequency raising, and so on. An oscillator is generally used in the phase-locked loop, frequency synthesizer and frequency generator circuits. A variety of methods are known for creating the oscillator circuits for providing periodic signals with a desired frequency.
The electronic oscillator is an electronic circuit that produces a repetitive electronic signal, often a sine wave or a square wave. FIG. 1 shows a circuit diagram of a conventional close loop oscillator. The close loop oscillator 100 satisfies Barkhausen Criteria, the loop gain satisfies A(jω0)β(jω0)=1, phase shift is 0(∠A(jω0)β(jω0)=0°), and it will oscillate at the frequency ω0.
The close loop oscillator circuit has a variety of applications, but is not limited to, such as the harmonic oscillator, relaxation oscillator, and crystal oscillator.
Firstly, the harmonic oscillator uses the self-oscillation to produce the sine wave without adding an external signal. The basic principal of the operation is to connect the output of a filter to an amplifier, and the output of the amplifier is coupled back to the filter's input. Therefore, those noisy signals are transmitted to the filter to filter out a portion of frequency and output them, and then the output signals is amplified by amplifier and entered into filter again and again, until the needed frequency is produced.
In practice, the harmonic oscillator can have different filtering methods and amplifies to realize the theory, like the Hartley oscillator, Colpitts oscillator, Clapp oscillator, Pierce crystal oscillator, phase shift oscillator, RC oscillator and Wien-Bridge oscillator, and so on.
In addition, relaxation oscillators or multivibrators are known in the monolithic integrated circuit design. The conventional relaxation oscillators operate by alternately charging and discharging a timing capacitor between two internally set threshold voltage levels. This results in the generation of a periodic output signal waveform whose frequency is inversely proportional to the capacitance value of the timing capacitor. The conventional relaxation oscillator configuration includes R-C charge and discharge oscillators, constant-current charge and discharge oscillators and emitter-coupled multivibrators.
Crystal oscillators have long been used to provide very accurate time keeping function because of their steady and predictable response to the physical or electrical stimuli. However, by their very nature, they do not have wide range of frequency.
For increasing the modulation capability of the oscillator, the researches usually use a modulated voltage to control the oscillator are populated. A voltage-controlled oscillator or VCO is an electronic oscillator designed to control the oscillation frequency by a voltage input. The frequency of oscillation is varied by the applied DC voltage, while the modulation signals may also be fed into the VCO to cause frequency modulation (FM) or phase modulation (PM); a VCO with digital pulse output may similarly have its repetition rate (FSK, PSK) or pulse width modulated (PWM).
Noticeably, no matter how progressive the oscillator is. The use of electric unit in the oscillator to charge or discharge naturally cannot avoid the noise, and always is affected by the input voltage, sometimes the characteristic of the frequency will change when the temperature varies. In FIG. 2, a conventional oscillator circuit is shown. An oscillator 200 includes an upper comparator 210, a lower comparator 220, an inverter 230, a discharge transistor 240, an inverter driven stage 250, a first resistor 260a, a second resistor 260b. a third resistor 260c, a first match resistor 270, a second match resistor 280 and a capacitor 290. Wherein all resistors are same, Vdd is input direct voltage, the upper comparator's 210 trigger voltage is ⅔ Vdd, the lower comparator's 220 trigger voltage is ⅓ Vdd. When the power is provided to the oscillator 200, the capacitor is charged through the first match resistor 270 and the second match resistor 280. The lower comparator 220 drives the inverter driven stage 250 to output a high level signal through a flip-flop 230, and turn-off the discharge transistor 240 to continue charging the capacitor 290. After the capacitor's 290 cross voltage is charged to ⅔ Vdd, the upper comparator 210 outputs a high level signal to make flip flop 230 to drives inverter driven stage 250 to output a low level signal, then the charge transistor 240 is turned on. The capacitor 290 is discharged through the second match resistor 280. After the capacitor's 290 cross voltage is discharged to ⅓ Vdd, the charge mode come back again.
Referring FIG. 2.1 is the relation of a conventional close-loop oscillator's Vc curve and Vo curve. The oscillator's 200 charge-discharge current and comparative voltage (⅓ Vdd & ⅔ Vdd) will change upon the Vdd, thus the frequency of oscillator 200 will change correspondingly.